Primary storage systems and caching appliances typically are used to store digital data associated with high-performance, networked computer systems. High-performance primary storage appliances generally include nonvolatile memory embodied in an integrated circuit, or semiconductor microchip, variously implemented in a storage memory array included on a circuit board in a computer or as a peripheral, or stand-alone, component. Caching appliances generally include a nonvolatile memory microchip, or set of microchips, located on or relatively near the central processing unit, or microprocessor chip, that provides relatively short-term temporary storage of data that has been relatively recently used by the processor or is likely to be used by the high performance data centers relatively soon.
In some existing configurations, or architectures, cache may refer to a dedicated, relatively high-speed, special-purpose microchip, while in other configurations cache may refer to a section or multiple segments of the general storage area apportioned for use as cache. In the latter case, the section or multiple segments of storage used as cache may remain static over time or may be dynamically reallocated among primary storage for caching over time.
Traditionally, relatively large, network-based storage devices have been implemented by magnetic hard-disk drives (HDD). More recently, some existing primary storage devices and caching appliances have implemented solid-state disk drives, such as “flash” memory that is made using arrays of NAND gates.
Existing single-level cell (SLC)-NAND integrated circuits have formed the basis for relatively high-speed primary storage and caching appliances. Given the relatively high performance, low power consumption, relatively fast read/write operation speeds, and endurance limit or life-cycle (reportedly as high as 100,000 program/erase cycles per cell), some all-flash-array primary storages made entirely or mostly of SLC-NAND memory chips have been implemented for use in industrial grade devices, embedded systems and critical applications, such as high-performance enterprise data centers, despite the relatively high cost of manufacturing these devices.
Some existing two-level, multi-level cell (MLC)-NAND integrated circuits can be manufactured at a relatively lower cost, but these also exhibit a significantly lower endurance limit or life-cycle (reportedly as high as 10,000 program/erase cycles per cell). Existing MLC-NAND integrated circuits have formed the basis for relatively high-speed storage in some consumer products.
Recently, the industry has taken interest in tri- or triple-level cell (TLC)-NAND integrated circuits, in significant part due to the relatively low cost of manufacture. TLC-NAND chips have a relatively high chip device density, providing up to three times the storage capacity of existing SLC-NAND chips, but exhibit slower read and write operation speeds than existing MLC-NAND memory chips. For example, TLC-NAND cells have been programmed in one-page (3×16 KB planes) or 3×16 KB (XP, UP, LP) planes, and read in three planes, one-by-one. The read/write latencies typically have been longer than MLC-NAND (two planes) and SLC-NAND chips.
In addition, existing TLC-NAND chips typically exhibit a relatively low endurance limit or lifecycle (on the order of 1,000 to 3,000 program/erase cycles per cell). Some TLC-NAND blocks can reportedly reach as many as 10,000 program/erase cycles when configured and used in SLC-mode, which, of course, reduces the memory capacity by two-thirds. In general, a block configured in SLC-mode to provide longer endurance could not be subsequently reconfigured back to TLC-mode to provide reliable storage capacity.
Thus, in some relatively demanding caching applications, for example, an existing 200 terabyte (TB) TLC-NAND SSD with a 200 Gbps write rate could wear out within six months of use. As a result, existing TLC-NAND memory chips typically have been limited to low-cost consumer storage devices, or commodity hardware, but typically have not been implemented in critical applications requiring frequent updating of data, such as high-performance enterprise data centers.